An energy-dispersive miniprobe multielement analyzer (EMMA) for direct analysis of trace elements and chemical age dating of single mineral grains

Abstract

An energy-dispersive miniprobe multielement analyzer (EMMA) was designed and constructed for the analysis of trace elements (As, Cr, Cu, Fe, Ga, Ge, Hf, Mn, Ni, Pb, Rb, Se, Sr, Th, Y, U, Zn) in small, individual mineral grains such as apatite, garnet, monazite, sphene, zircon, and other accessory minerals. The instrument is sensitive (ppm for most elements), rapid (∼ 3–6 min per analysis), non-destructive, and essentially no sample preparation is required. With respect to single zircon grains as small as 50 μm with mass as low as a few μg, for example, the detection limits for U, Th, and Pb are on the order of 10 ppm (5–10 times better than the electron microprobe). Here, the EMMA is used to illustrate the variation in Hf and Y concentrations in single zircon grains from pegmatites, granites, basalts, carbonatites, and kimberlites. Also, the instrument is used for chemical age dating (U,Th-total Pb) of: (1) single zircon grains from the Berdyshevsky intrusive complex (Ukraine); and (2) single monazite grains from a small placer deposit within the transition zone between the Saxothuringikum and Moldanubikum (Bavaria, Germany). The average age of the single zircon grains from Berdyshevsky granite determined using EMMA (2.19±0.17 Ga) (1σ) is consistent with the UPb zircon intrusion age of this complex (2.14±0.07 Ga) and monazite data ranging from 1.95 to 2.48 Ga. With respect to the pegmatite from this complex, the average age of 1.93±0.09 Ga found with EMMA is geologically consistent with zircon and monazite ages ranging from 1.90 to 2.29 Ga. Regarding the monazite grains from the placer, the age dates obtained using EMMA (328±28 Ma) are comparable to the isotopic ages (conventional UPb and PbPb step leaching) which yielded ages of 312 ± 3 and 324 ± 12 Ma, respectively.